One known energy-saving high-voltage semiconductor device is a power transistor that includes a wide band-gap semiconductor. One example of such a wide band-gap semiconductor is GaN. GaN has a band gap of 3.4 eV, which is higher than the band gap of common semiconductor materials Si (1.2 eV) and GaAs (1.4 eV).
A power transistor that includes a wide band-gap semiconductor GaN has a high breakdown voltage, which allows the distance between electrodes and on-resistance to be reduced. Transistors having reduced on-resistance generate less heat and are therefore energy-saving devices.
Present power devices contain Si as a main semiconductor material. Specific examples of such power devices include power transistors, insulated gate bipolar transistors (IGBTs), and cool metal-oxide-semiconductor field-effect transistors (MOSFETs), each containing Si. GaN transistors have on-resistance an order of magnitude lower than that of Si transistors and therefore have potential for high-voltage high-efficiency power devices for use in switching power supplies and electric vehicle inverters.
It is desirable that power transistors for use in switching power supplies not only have low on-resistance but also are capable of normally-off operation, during which no electric current flows unless a control voltage, such as a gate voltage, is applied. Thus, the structure of GaN transistors capable of normally-off operation has been studied. One example of GaN transistors capable of normally-off operation is a vertical metal-insulator-semiconductor field-effect transistor (MISFET) that includes a p-GaN layer.
However, a high concentration of acceptor in a p-GaN layer is difficult to activate. Furthermore, use of a simple GaN bulk layer as an electron transit layer (drift layer) is not achieve high electron mobility, unlike two-dimensional electron gas (2 DEG) in high-electron-mobility transistors (HEMTs). This results in high on-resistance.
One example of vertical GaN-FETs having a normally-off structure without using a p-GaN layer, which has difficulty in activating a high concentration of acceptor, has a GaN/AlGaN heterojunction barrier.
However, even with the GaN/AlGaN heterojunction barrier, use of a GaN bulk layer as an electron transit layer still increases on-resistance.    [Patent Document] Japanese Laid-open Patent Publications No. 2008-192701    [Patent Document] Japanese Laid-open Patent Publications No. 2008-53448    [Patent Document] Japanese Laid-open Patent Publication No. 2011-91109